For protection of a series capacitor it is known to arrange, in parallel with the capacitor, a voltage-dependent resistor means built up of metal oxide varistors in the form of ceramic blocks. A protective spark gap provided with means for forced triggering is usually arranged in parallel with this resistor means. The cost of the resistor is kept low by dimensioning the resistor to absorb only a limited amount of energy. When the absorbed energy exceeds the level which is predetermined by the dimensioning, the parallel spark gap is activated and is later extinguished by external interruption of the current or shunting. Various triggering devices for a protective spark gap of the kind mentioned are known. In a device disclosed in U.S. Pat. No. 4,174,529 the energy for triggering the spark gap is taken from a current transformer arranged in series with the resistor. Since the triggering may reguire a relatively high power, this trsnsformer will have a high rated power and is therefore expensive. In other known devices (see U.s. Pat. No. 4,625,245), special triggering circuits with auxiliary spark gaps of the kind used in older surge arresters with silicon carbide varistors are used. Since modern surge arresters are built up of zinc oxide varistors without series-connected spark gaps, the need for these spark gaps has considerably decreased. This has led to a situation in which there is no longer any market for a rational manufacture of such spark gaps. The aim is therefore to avoid the use of auxiliary spark gaps also in protective circuits of the kind to which the present invention relates.